PROJECT SUMMARY (Seeinstructions): The overall objective of the proposed research is to delineate, and make sense of, the process of lexical development and its relations to socio-communicative development, grammatical development, and both general and specific aspects of nonlinguistic cognitive development. The research will focus on the language and cognitive development of children with Williams syndrome and children with Down syndrome. Williams syndrome is associated with relative strengths in language (especially concrete vocabulary and grammar) and verbal short-term memory and severe weakness in visuospatial cognition. In contrast, Down syndrome is associated with severe weaknesses in expressive language (especially grammar) and in verbal short-term memory and relative strength in nonlinguistic aspects of cognition. Because of these differences in the general nature of the relations between language and nonlinguistic cognition, inclusion of these two groups of children in the same studies provides a unique opportunity to distinguish factors that are likely to be universal and necessary to language development from those that are specific to particular rates or paths to language development. The proposed research consists of a longitudinal study of early language and cognitive development beginning in late infancy, a longitudinal study of cognitive and language development of school-age children as measured by standardized assessments, and cross-sectional studies of language development, especially socio-communicative development. Both observational and experimental methods will be used. There are three specific aims. The first is to delineate the different types of early lexical growth curves and examinetheir relations to later patterns of language and cognitive development. The second is to examine socio-communicative development and its role in language and cognitive development. The third is to determine the longitudinal trajectories of language and cognitive abilities of children with Williams syndrome and factors that influence these trajectories. The researchwill have implications both for theoretical models of the relations between language and cognitive development and for the design of cognitive, language, and reading intervention for children with developmental or intellectual disabilities.